Control of a boom construction and a tool articulated thereto

ABSTRACT

A method for controlling a boom construction and a tool articulated thereto, the boom construction comprising a first boom and a first actuator controlling the same, a second boom articulated to the first boom, and a second actuator controlling the same, as well as a third actuator controlling the position of the tool, wherein in the method: the first actuator is controlled manually; and/or the second actuator is controlled manually and the third actuator is also controlled at the same time automatically to control the position of the tool. For example, the first actuator is controlled by use of a first set point; and/or the second actuator is controlled by use of a second set point; and in addition, the third actuator is controlled simultaneously by using for the control a variable dependent on at least one of said set points. For example, the control system of the boom construction and the tool articulated thereto comprises a control algorithm, which has been configured to determine the variable which is dependent on at least one of the set points.

FIELD OF THE INVENTION

The invention relates to a method for controlling a boom constructionand a tool articulated thereto. The invention also relates to a systemfor controlling a boom construction and a tool to be articulatedthereto.

The invention relates to hydraulically controlled boom systems, to whicha felling head is coupled, particularly a boom system for a fellerbuncher, and its control.

BACKGROUND OF THE INVENTION

Felling heads are widely used for the cutting and collecting of uprightgrowing trees. In most cases, several tree trunks can be collected inthe felling heads until the tree trunks are unloaded onto the ground.After this, the tree trunks are transported elsewhere by means of eithera forwarder or a skidder. The boom system and the felling head arenormally coupled to a high-power vehicle, such as a feller buncher, ofwhich an example is given in U.S. Pat. No. 6,173,973 B1. Some fellingheads are disclosed in documents US 2003/02025291 A1 and US 2002/0040741A1. A felling head and a feller buncher are also disclosed in Canadianpublication 2,196,728 A1. Vehicles provided with a horizontal movementof the boom are disclosed in publications U.S. Pat. No. 6,443,196 B1 andU.S. Pat. No. 6,997,221 B2.

The feller buncher normally comprises a lower carriage and an uppercarriage. The lower carriage is equipped, for example, with twocaterpillar tracks, by means of which the forest machine moves on aterrain. Above the traverser of the lower carriage, an upperconstruction is placed, comprising, inter alia, a cabin, an engine andan articulated boom construction. The boom construction has for exampletwo parts, comprising a hoist boom and a stick boom, the felling headbeing articulated at the end of the stick boom.

The cabin of the feller buncher is provided with two control levers orcontrol handles for the operator. The control levers are typicallymovable in two directions: forward-backward (y-axis) and to theleft-right (x-axis). By means of the y-direction, one of the two controllevers controls the position of the hoist boom (hoist function) and theother the position of the stick boom (stick function). In acorresponding manner, by means of the x-direction, one of the controllevers typically controls the rotation of the upper carriage and theother the position of the felling head.

For bringing the felling head to the tree to be felled, the drivernormally has to operate both of the control levers in the y-direction(hoist and stick functions). Because the movement of the hoist boomand/or the stick boom continuously changes the position of the fellinghead with respect to the ground, at least one of the control levers mustalso be moved in the x-direction. For felling the tree, the felling headis equipped with, for example, a saw that must be kept in the horizontalplane. A corresponding simultaneous operation of the control levers isrequired when the tree has been gripped in the felling head and thefelling head together with the tree is moved towards new trees to befelled, or when a collected bunch of trees is unloaded onto the groundin a controlled manner for emptying the felling head. Consequently, inpractice, the driver must almost all the time control said hoist andstick functions and the position of the felling head simultaneously, andmoreover, he typically rotates the upper carriage. The control of theboom construction and the keeping of the felling head in the correctposition require significant alertness and skill of the operator. Thesimultaneous manipulation of several control levers is difficult and mayslow down the function.

SUMMARY OF THE INVENTION

The method according to the invention for controlling the boomconstruction and the tool articulated to it is presented in the claimssection herein. The system according to the invention for controllingthe boom construction and the tool articulated to it is presented in theclaims section herein.

Thanks to the invention, the operator does not need to separately takecare of changing the position of the felling head, when the position ofthe hoist boom and/or the stick boom is changed. The control system willautomatically take care of maintaining the position of the felling head,for example, upright when the position of the hoist boom and/or thestick boom is changed. The control system allows the operator to changethe position of the felling head.

The invention makes the work of the operator easier and faster. With theinvention, the need for the operator to control the position of thefelling head by an x-movement of one of the two control levers willbecome minimal. In practice, keeping the felling head straight, whichrequires accuracy and skill, will be a fully automatic function. Theoperator will only use the x-movement of the control lever primarily forunloading the bunches of trees collected in the felling head and forreturning the felling head to the upright position.

The manipulation of the control levers can be simplified further, whenthe position of both the hoist boom and the stick boom is simultaneouslycontrolled automatically by using only one control lever. This functionis used for achieving a substantially linear movement of the head of thestick boom. Now, the felling head remains, for example, at a constantheight when it is moved farther away or closer to the forest machine bymeans of both the hoist boom and the stick boom. The function is calleda horizontal or parallel movement which is in most cases substantiallyhorizontal and which is controlled, for example, by the control levercontrolling the position of the hoist boom. In addition to the parallelmovement, the control system also allows the operator to change theposition of the hoist boom. In this way, the motion angle or height canbe easily adjusted, if necessary, by the control lever for the hoistmovement.

The automatic control of the position of the felling head and thefunctions of the parallel movement can be combined. Thus, themanipulation of the machine becomes very easy. By only one movement ofthe control lever (for example, the y-axis normally controlling thestick boom), the felling head can be driven outwards and inwards in thehorizontal direction while the felling head remains in a set position,for example upright.

By means of the invention, it is possible to automate and combinefunctions, wherein the manipulation and operation of the boom systemwill be simpler and faster. The implementation is electronic and allowseasy modification. The invention makes it possible to implement thelevelling of a tool, for example the felling head, without sensors formeasuring the position of the felling head, or at least the need forthem is substantially reduced.

The invention can be applied in various boom constructions in whichtools are fixed by articulation so that the tools can be turned around asubstantially horizontal line by means of a working cylinder fixed, onone hand, to the boom construction and, on the other hand, to the toolor an adapter for articulating the tool at the end of the boomconstruction. Applications include particularly forest machines,harvesters, forwarders or combinations of them, in which for example athinning and collecting tool according to application FI-910197, or acollecting tool functioning on a corresponding principle, is connectedto the end of the boom construction for the collection of fuel wood. Thetool typically comprises a frame, to which arms are articulated forcollecting one or more tree trunks, and a cross-cutting device forcross-cutting the tree which has been clamped. In most cases,cross-cutting devices operating by the guillotine principle or chainsaws are used.

The invention can be applied, for example, in excavators, forcontrolling either a felling head that is available as an accessory forthem, or the actual bucket. In excavators, the boom construction, thelower structure and the upper structure correspond to feller bunchersrather accurately.

The invention can also be applied in boom constructions of forestmachines, in which the structure is configured solely for the parallelmovement. The boom construction comprises a transfer cylinder which iseffective, for example by means of an articulated mechanism, on theposition of the two booms in the boom construction. The control systemwill automatically take care of the position of the tool when theposition of the booms is changed.

The structure of the boom construction, the placing of the correspondingactuators, and the articulated mechanism related to the actuator mayvary in boom constructions applying the invention. The placement andfunction of the actuator correspond, for example, to the solution of WO01/02280 A1, in which a parallel movement is also achieved by theactuators.

The other advantages and the details of the invention will be presentedin the more detailed description and the drawings of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in a side view, a forest machine which is a feller buncherand in which the invention is applied, the felling head being close tothe forest machine,

FIG. 2 shows, in a side view, the forest machine of FIG. 1 in asituation in which the felling head has been transferred far away fromthe forest machine by means of the boom construction,

FIG. 3 shows components and the principle of operation of the controlsystem of the forest machine in a reduced chart,

FIG. 4 illustrates the more detailed operation and the controlprinciples of the control system of FIG. 1 in a simplified diagram, and

FIG. 5 illustrates the operation of the control system of FIG. 3 in moredetail and its control principles in a flow chart.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, we shall describe in more detail the way ofcontrolling the boom construction by means of the control system whenthe boom construction is coupled to a motor vehicle which is a workingmachine. Thus, a suitable tool is coupled to the end of the boomconstruction for accomplishing the work.

FIGS. 1 and 2 show a vehicle which is a forest machine and particularlya feller buncher 1. The feller buncher comprises a lower carriage 2, bymeans of which the feller buncher moves on a terrain. The lower carriage2 is equipped with for example two caterpillar tracks. On top of thetraverser of the lower carriage 2, an upper construction 3 is placed,comprising the cabin and the engine. A boom construction 4 isarticulated to the upper structure 3. By means of the traverser, theupper structure 3 and the boom construction 4 can be rotated around avertical line. The boom construction typically comprises two booms,which are also called a hoist boom and a stick boom in this example. Inboom constructions intended for a parallel movement, the boom may alsobe called a transfer boom. In FIGS. 1 and 2, the boom construction 4 hastwo parts, comprising a hoist boom 9 and a stick boom 8, the fellinghead 5 being coupled to the end of the stick boom 8. The felling head 5is used as the tool of the working machine.

For the movement, the boom construction comprises actuators which inthis example are working cylinders and which are also called a hoistcylinder and a stick cylinder. In boom constructions intended for theparallel movement, the working cylinder may also be called a transfercylinder. The end of the hoist boom 9 is coupled by means of anarticulation to the upper structure 3, and the outermost end of thehoist boom 9 can be steered up and down. The hoist boom 9 can be movedby one or two hoist cylinders 10 which are fixed by articulation to boththe hoist boom 9 and the upper structure 3. The hoist cylinder 10 is adouble-acting cylinder whose length, speed of movement and direction ofmovement can be controlled by means of a pressurized medium, normallyhydraulic oil. The medium is a manipulated variable which is affected bya controlling element, such as a valve.

The end of the stick boom 8 is fixed by articulation to the outermostend of the hoist boom 9, and the outermost end of the stick boom 8 canbe steered up and down, and simultaneously also forward and backward.The stick boom 8 is moved by a stick cylinder 11 which is fixed byarticulation to both the hoist boom 9 and the stick boom 8. The stickcylinder 12 is a double-acting working cylinder 11 whose length, speedof movement and direction of movement are controlled by the pressurizedmedium. The booms are coupled to each other by a link mechanism 4 a, towhich the stick cylinder 11 also belongs. The working cylinder iscoupled to the boom either directly or by means of a suitable linkmechanism.

The felling head 5 is coupled by an articulation 12 at the end of thestick boom 8, and the felling head 5 can be tilted forward and backwardaround a horizontal line extending via the articulation 12. At the endof the stick boom 8, an adapter 6 for the boom construction is provided,coupled to the felling head 5 and tilting with the felling head 5 aroundthe articulation 12. The felling head 5 is controlled by a tiltingcylinder 7 that is articulated to both the stick boom 8 and the fellinghead 5, in the presented example by means of the adapter 6. The tiltingcylinder 7 controls the position of the adapter 6 and simultaneously thetilting of the felling head 5. The tilting cylinder 7 is a double-actingcylinder whose length, speed of movement and direction of movement canbe controlled by means of the pressurized medium.

The felling head 5 can also be rotated, for example, around a horizontalline by means of a rotating device fixed between the adapter 6 and thefelling head 5. Said horizontal line extends transversely, normallyperpendicularly to the line extending via the articulation 12. Theadapter 6 and the rotating device also form a part of the felling head5.

The feller buncher 1, the boom construction 4, the working cylinders 7,10 and 11, as well as the felling head 5 are known as such with respectto their structure, and the principles applied in them are known to aperson skilled in the art. In this description, we shall discussfeatures which are not known in prior art. The operating principles ofthe control system of the feller buncher 1 are also known as such, butnovel principles according to the invention are applied therein. Themodification and configuration of the control system will be obvious fora person skilled in the art on the basis of the description of theinvention in this specification.

As shown in FIG. 1, the felling head 5 can be tilted by the tiltingcylinder 7 for example from position P1 to position P2 around thearticulation 12, wherein the tilting cylinder 7 becomes longer when itmoves. As shown in FIG. 2, the felling head 5 can be moved by means of aparallel movement from a location L1 to a location L2 along asubstantially horizontal path 13, wherein the hoist cylinder 10 and thestick cylinder 11 act simultaneously. Because the hoist cylinder 10 andthe stick cylinder are mounted under the boom construction 4, the hoistcylinder 10 becomes shorter and the stick cylinder becomes longer. Theplacement of the working cylinders may also vary, wherein the stickcylinder may also be mounted on top of the boom construction 4, in whichcase the stick cylinder becomes shorter when the felling head 5 istransferred to the location L2. When the felling head 5 is in thelocation L1, the boom construction 4 is in the position shown in FIG. 1.During the transfer, the felling head 5 is kept, for example, in anupright position P1 by controlling the tilting cylinder 7. The tiltingcylinder 7, the hoist cylinder 10 and the stick cylinder 11 operatesimultaneously.

FIG. 3 shows in more detail the operation of the control system 21 forcontrolling the controlling elements 22, 23 and 24. The controllingelements act on a manipulated variable which in this case is a mediumthat is led in and out of the working cylinders. The pressure and volumeflow of the medium, in turn, control the speed of the working cylinders7, 10 and 11. The working cylinders, in turn, steer the boomconstruction 4 into the desired position. Set points 15 and 16 are inputin the presented control system 21, the position and movements of theboom construction 4 being dependent on the set points. The operatorenters the set points manually by using the necessary equipment,particularly by manipulating control sticks 19 and 20, whose directionsof movement correspond to the directions of movement of the workingcylinders 7, 10 and 11. In the neutral position of the control stick,the working cylinders remain stationary.

The control system 21 has a link mode, wherein the control stick 19 isused for controlling the stick cylinder 11. For entering the set point,movements in the y-direction are used to make the stick cylinder 11either shorter or longer. In this example, a set point is also given forthe tilting cylinder 7 by means of the control stick 19, whereinmovements in the x-direction are used to make the tilting cylinder 7either shorter or longer. The control stick 20 is used for controllingthe hoist cylinder 10, wherein movements in the y-direction are used. Inthe presented example, a forward movement of the control stick 20 willlower down the boom construction 4, and a backward movement will hoistthe boom construction 4. If desired, the control stick 20 has also amovement in the x-direction, which is normally applied for controllingthe rotation of the upper carriage. From the above-described controlarrangement, it is also possible to obtain, for example, a modificationthat the y-directions of the control sticks 19 and 20 control thefunctions crosswise; in other words, the control stick 19 controls thehoist function and the control stick 20 controls the stick function.

In addition to the link mode, the control system 21 also has a parallelmode for the parallel movement, which is activated e.g. by a button 25provided either in the control stick 19, in its direct vicinity orelsewhere within the reach of the operator. In the parallel mode, thecontrol stick 19 and its set point are used to control both the hoistcylinder 10 and the stick cylinder 11 simultaneously so that, forexample, the movement along the route 13 is possible by using only onecontrol stick 19. A forward movement of the control stick 19 in they-direction will transfer the felling head 5 farther away, and abackward movement will bring the felling head 5 closer to the upperstructure 3. Normally, the hoist cylinder 10 can be controlled using they-direction of the control stick 20 also during the parallel movement.It is also possible to obtain, for example, such a modification of theabove-described control arrangement that the y-directions of the controlsticks 19 and 20 will control the functions crosswise; in other words,the control stick 19 will control the hoist function and the controlstick 20 will control the parallel movement.

The control system 21 also comprises a felling head levelling mode whichis activated e.g. with a button 29 which may be provided in the controlstick, in its direct vicinity or elsewhere within the reach of theoperator. In the felling head levelling mode, the function of thetilting cylinder 7 depends, among other things, on the set points 15 and16. The operation of the tilting cylinder 7 is controlled on the basisof signals from the control sticks 19 and 20 on the hoist and stickfunctions, and on the position of the felling head. The felling headlevelling mode is available in both the link mode and the parallel mode.When both the felling head levelling mode and the parallel mode are inuse, all the working cylinders 7, 10 and 11 can be jointly controlledsolely by the y-movement of the control stick 19. The felling headlevelling mode can also be taken into use in a control system with noparallel mode.

FIG. 3 also shows in more detail the controlling elements 22, 23 and 24which are in contact with the medium system comprising a pressure line17 containing pressurized medium and a return line 18 to which themedium is returned. The medium system is known as such, and it is usedto produce the pressure and volume flow of the medium. The medium is amanipulated variable whose entry into the working cylinders 7, 10 and 11is controlled by the controlling elements 22, 23 and 24. The variable tobe controlled is the speed and the direction of movement of the workingcylinder, and the controlling variable is e.g. a current signal 26, 27or 28 obtained from the control system 21. The system may also compriseseveral separate pressure lines and return lines. The controllingelements 22, 23 and 24 are provided for the tilting cylinder 7, thehoist cylinder 10 and the stick cylinder 11, respectively. In thepresented example, the controlling element is a valve controlled by acurrent signal and providing a volume flow proportional to the currentsignal to be supplied to the working cylinder. The structure of thecontrol system 21 may differ from that presented, and it may be moredistributed, or it may form a part of a larger control system. Thecontrol system is, for example, a PC based control system which is knownas such and in which the principles of the invention are applied, andwhich also comprises the applications required for e.g. the controlalgorithms. The valves are shown on the level of principle and theycomprise, for example, electronically controlled three-positionedfour-way directional valves. The functions can also be implemented byproportional directional valves. A control and amplifier card coupled tothe proportional valve can be used to set e.g. the necessaryacceleration and deceleration ramps in a way known as such.

The equipment of the control system comprises, for example, a displaymodule, a PC keyboard and a central unit with a processor and a memory.For implementing the various examples of the invention, the requiredapplication and the software included therein are installed in thecentral processing unit of the control system which comprises thenecessary RAM and mass storage. The control system utilizes an operatingsystem known as such, under which the application is run. The equipmentand the operating system comprise the applications and protocolsnecessary for communication with other devices. The operating system ispreferably a turnkey system of prior art, which even provides servicesready for the transmission of a data stream in e.g. a CAN bus(Controller Area Network).

FIG. 4 shows the principle of operation of a control algorithm of thecontrol system which is illustrated by means of inputs 30, 31 and 32corresponding to the set points of different movements as well as bymeans of an output 34 corresponding to the controlling variable. Thecontrolling variable is entered in the controlling element and it isused, among other things, for controlling the tilting of the fellinghead. The control algorithm utilizes stored parameters that can be set.This will be discussed in more detail hereinbelow.

FIG. 4 shows in more detail particularly the operation of the fellinghead levelling mode, and the other modes operate as will be presentedhereinbelow.

We shall first discuss situations in which the felling head levellingmode is not in use. In the link mode, the output 34 that controls thetilting cylinder 7 (tilt_command) is only proportional to the input 31(tilt_joystick) corresponding to the set point given by means of thex-movement of the control stick 19 (e.g. tilt_command=tilt_joystick). Ina corresponding manner, in the link mode and in the parallel mode, thestick cylinder 11 is controlled by an input 30 (stick_joystick,sl_joystick) that corresponds to the y-movement of the control stick 19(e.g. stick_command=stick_joystick, sl_command=sl_joystick). In acorresponding manner, in the link mode and in the parallel mode, thehoist cylinder 10 is controlled by an input 32 (hoist_joystick) thatcorresponds to the y-movement of the control stick 20 (e.g.hoist_command=hoist_joystick). Furthermore, the hoist cylinder 10 iscontrolled by the input 30 (sl_joystick), when the parallel mode is inuse (e.g. hoist_command=hoist_joystick+sl_joystick). If desired, thelink mode and the mode of the parallel movement can be supplemented withparameters and control means whose principles have been described inconnection with the felling head levelling mode.

In the felling head levelling mode, the control algorithm and its output34 (tilt_command) can be described generally in the form:tilt_command=tilt_joystick+(ff_hoist*hoist_joystick)+(ff_stick*stick_joystick)+(ff_sl*sl_joystick).

In the link mode, when the felling head levelling mode is in use, theoutput 34 that controls the tilting cylinder 7 (tilt_command) isproportional to the input 30 (stick_joystick) and the parameter ff_stick(e.g. ff_stick*stick_joystick), as well as to the input 31(tilt_joystick) and also to the input 32 (hoist_joystick) and theparameter ff_hoist (e.g. ff_hoist*hoist_joystick). The parametersff_stick and ff_hoist are parameters that can be set and some kind ofamplifications.

In the parallel mode, when the felling head levelling mode is in use,the control algorithm operates otherwise as described above, except thatthe input 30 is now sl_joystick, with which the parameter ff_sl is used(e.g. ff_sl*sl_joystick) and which normally differs from the link modeparameter ff_stick. Even though the same control stick 19 is used in theparallel and link modes, the set point 30 varies depending on theselected mode, and the selection of the controlled controlling elementsvaries.

A summing unit 35 indicates the combined effect of the inputs, and thecontrol member 33 indicates, for example, readjustments related e.g. topossible maximum and minimum values of the output 34. In connection withthe control member 33, it is also possible to take other measuresrelating to the control algorithm, which are considered necessary orwhich fine-tune the operation.

According to the above example, in the felling head levelling mode, they-movements of the control sticks 19 and 20 (hoist and stick functions)normally affect the position of the boom construction 4 but now also theposition of the felling head 5. When running the hoist or stick function(link mode) or the parallel movement combined of these (parallel mode),the control system takes care of the position of the felling head 5automatically, but in this example, it is also possible to correct theposition of the felling head 5 manually with an x-movement of thecontrol stick 20 (tilt_joystick). The control system operates so thatthe aim is to maintain the latest position of the felling head set bymanual control when using the boom construction.

In the apparatus according to the example, the parallel movement can beachieved by a control algorithm, wherein on the basis of the input 30(sl_joystick), both the hoist cylinder 10 and the stick cylinder 11 aremoved simultaneously by controlling both the controlling element 23 andthe controlling element 24 with variables that are proportional to theinput 30.

Alternatively, according to prior art, the hoist cylinder 10 and thestick cylinder 11 can be coupled functionally in series by a controllingelement so that e.g. the controlling element 24 alone controls both thehoist cylinder 11 and the stick cylinder 11. Thus, the controllingelement 24 is controlled with a suitable variable which is proportionalto the input 30.

For the parallel movement, according to prior art, boom constructionsare provided, to which the working cylinder is coupled so that theworking cylinder is effective on the position of two booms. Typically,the boom construction is a link mechanism, by means of which the workingcylinder is effective on both booms. Furthermore, another workingcylinder is effective on one of the booms for hoisting. The boomconstruction has a parallel mode but not a link mode.

Good and versatile automatic control of the felling head is achieved bydefining at least one parameter for the felling head levelling mode foreach direction of movement of the control stick (forward, backward). Theparameters can be defined to deviate from the linear to obtain a controlthat is unlinear in a desired manner, taking into account for exampleunlinearity or behaviour of the controlling element. Consequently, theparameter may be a function in which the control value is entered.

Thus, the parameter ff_hoist can be divided into two parametersff_hoist_up and ff_hoist_down, depending on the side on which thecontrol stick 20 is in relation to the neutral position. When hoistingthe hoist boom 9, ff_hoist_up is used, and the felling head 5 is tiltedforward by maintaining its position, for example P1. In a correspondingmanner, when the hoisting boom 9 is lowered down, ff_hoist_down is used,and the felling head 5 is tilted backward.

The parameter ff_stick of the link mode can be divided into twoparameters ff_stick_out and ff_stick_in, depending on the side on whichthe control stick 19 is in relation to the neutral position. Forexample, if the end of the stick boom 8 is transferred farther away andff_stick_out is in use, then the tilting head 5 is tilted forward sothat the position of the felling head 5 is maintained, for example theposition P1. In a corresponding manner, if the end of the stick boom 8is brought closer to the upper structure 3 and ff_stick_in is used, thenthe felling head 5 is tilted backwards.

In the parallel mode, the parameter ff_sl can be divided into twoparameters ff_sl_out and ff_sl_in, depending on the side on which thecontrol stick 19 is in relation to the neutral position, that is, theposition in which the input/control value is zero. For example, when thefelling head 5 is transferred farther away along the path 13, from thelocation L1 to the location L2, the parameter ff_sl_out is used and thefelling head 5 is automatically tilted forward so that the position ofthe felling head 5, for example P1, is maintained. In a correspondingmanner, when the felling head 5 is brought closer, the parameterff_sl_in is in use and the felling head 5 is tilted backwards.

In this elucidating example, the parameters obtain values between aminimum value and a maximum value, for example from 0 to 100% (from 0.0to 1.0). For example, if the value of the parameter ff_hoist_up is 50%and the y-direction of the control stick 20 gives a maximum set point100% or the maximum input 32 for the hoist function, then thecontrolling variable of the tilting cylinder 7 or the output 34 is 50%of the maximum. In other words, the output 34 gives for example a 100%control to the controlling element 23 for moving the working cylinder 10and a 50% control to the controlling element 22 for moving the workingcylinder 7. With the parameters, the desired ratios are set for thespeeds of movement of the working cylinders to keep the felling head 5in the desired position.

If desired, the operation of the control algorithm for the levelling ofthe felling head can be easily adjusted with the parameters. Forexample, when the felling head levelling mode is activated, it can bedeactivated for example in connection with a control stick or adirection of movement. The adjustment is made by setting the desiredvalues for the parameters. For example, if it is desired that thelevelling of the felling head is only active when running out but notrunning in the parallel movement, the parameter ff_sl_in is set to zero.In a corresponding manner, if, when running in the parallel movement,the levelling is to be activated only partly, i.e. by underlevelling,the value of the parameter ff_sl_in is set between zero and a parametervalue giving full levelling. When the operation is to be overlevelling,the value of ff_sl_in is set greater than the parameter value givingfull levelling.

In FIG. 4, the output 34 shows the control obtained by the controlvalue, wherein 0% represents a closed valve and 100% represents a fullyopen valve. The output tilt_command is limited between −100% and +100%,wherein the negative direction and the positive direction representtilting in different directions, starting for example from the positionP1. The output −100% to 0% represents the first position of the valveand the output 0% to 100% represents the second position of the valve.When the output is 0%, the valve is in the central position shown inFIG. 3. Typically, the output tilt_command and also the outputs of theother movements are provided with ramp functions relating to thestarting and stopping of the working cylinder, wherein for example thetilting cylinder 7 responds quickly to the movement of the control stick19; that is, the ramp is short. The ramp used in connection with theparallel movement, in turn, may be long, wherein the movements are moregentle.

Mutual dependencies can be determined between outputs, for examplebetween the outputs tilt_command and tilt_ff_command. For example, theoutput tilt_command can be prioritized in situations in which thecontrol system detects that the driver is trying hard to control thetilting of the grapple manually in another direction than the controlalgorithm. The aim of the prioritizing is to guarantee that if desired,the driver can in all situations use manual control to obtain a maximalspeed of motion for the tilting of the felling head.

FIG. 5 shows the operation of the control algorithm in more detail in aflow chart. The first step 38 of the procedure 36 is to calculate theoutput tilt_command that controls the tilting cylinder 7 and isproportional only to the input tilt_joystick of the manual control ofthe tilting of the felling head. The next step 37 is to examine if thefelling head levelling mode is activated. If the levelling is notactivated, the output already calculated is transferred further to aregulating element and the procedure returns via step 41 to the step 38.If the felling head levelling mode is in use, the next step will be step39 in which the output tilt_command controlling the tilting cylinder 7is also proportional to the dependence tilt_ff_command defined on thebasis of the other inputs. From the manual control (tilt_joystick) andthe levelling control (tilt_ff_command), one combined output can becalculated, which is transferred to the controlling element, and theprocedure returns along line 40 to the step 38. The control systemexamines the status of the inputs and the activated mode continuouslyand determines continuously the presented dependence between the setpoints and the controlling variable.

Some examples of the invention have been described above in detail.Modifications and variations of the examples will be obvious for aperson skilled in the art on the basis of the above description. Theinvention is not limited solely to the examples presented above, but itis defined according to the appended claims.

Having described the preferred embodiment, it will become apparent thatvarious modifications can be made without departing from the scope ofthe invention as defined in the accompanying claims.

The invention claimed is:
 1. A method for controlling a boomconstruction and a tool, the boom construction comprising a first endand an opposite second end, the first end connected to a working machineby articulation, the tool connected to the second end by articulation,the boom construction comprising a first boom and a first actuatorcontrolling the same, a second boom articulated to the first boom and asecond actuator controlling the second boom, and a third actuatorcontrolling the position of the tool, wherein the method comprises thesteps of: a control system selectively controlling simultaneously thefirst actuator with a first set point and the second actuator with asecond set point in a link mode so as to control the first and secondactuators in a manner independent of one another or controllingsimultaneously the first and second actuators with the first set pointand a third set point in a parallel mode for a parallel movement of thesecond end of the boom construction, and the control system controllingthe third actuator in a tool-levelling mode that targets positioning thetool in a constant attitude during movement of the second end of theboom construction by using a variable dependent on the first and secondset points when controlling the first and second actuators in the linkmode and by using the variable dependent on the first and third setpoints when controlling the first and second actuators in the parallelmode.
 2. The method according to claim 1, characterized in the controlsystem controlling the third actuator in the tool-levelling mode with afourth set point for tilting the tool forward or backward.
 3. The methodaccording to claim 2, characterized in that said variable is jointlyproportional to the first, second and fourth set points when the linkmode and the tool-levelling mode are active and jointly proportional tothe first, third and fourth set points when the parallel mode and thetool-levelling mode are active.
 4. The method according to claim 1,characterized in setting said set points by one or more manual controlsticks.
 5. The method according to claim 1, characterized in the controlsystem controlling the third actuator with a controlling element, onwhich said variable is effective and which controls the volume flow of amedium to the third actuator.
 6. The method according to claim 1,characterized in the control system controlling the third actuator inthe tool-levelling mode with a fourth set point for tilting the toolforward or backward, said variable is jointly proportional to the first,second and fourth set points when the link mode and the tool-levellingmode are active and jointly proportional to the first, third, and fourthset points when the parallel mode and the tool-levelling mode areactive.
 7. The method according to claim 1, characterized in manuallyinputting the first, second, and third set points.
 8. The methodaccording to claim 1, characterized in that the too; is a felling head.